This confusion is also something I see frequently while proofreading. In fact, I have just completed proofreading an article where it was spelled correctly only once out of twelve incidences of the word over two pages!
Principal is a noun meaning ‘chief’ or ‘most important’ person, such as in ‘the principal [head teacher] of the school’ or as an adjective, as in ‘the principal export is gold’.
Principle is a noun meaning ‘general truth’, ‘law’, or ‘rule’, such as in ‘he refused to abandon his ethical principles’.